The present invention relates to an optical pickup device for an optical magnetic recording/playback apparatus, and more particularly to an optical pickup device adapted to irradiate a laser beam to a recording medium such as an optical disk, etc. to record information onto the magnetic medium, or to play back or reproduce the information therefrom.
Generally, since the signal detection optical system of an optical pickup for an optical magnetic recording/playback apparatus is complicated, the weight of the entirety of the pickup is increased, resulting in a major problem in realizing a high speed access. As the means for solving this problem, there is known a separation type optical system such that only an objective lens of an optical pickup, an actuator unit for driving the lens, and a beam directing prism are mounted on a feed mechanism for directing the laser beam to the lens, to thus construct a movable unit transferred in a radial direction of the disk, and that other portions of the optical pickup are fixed to the base plate of the recording/playback apparatus to thereby reduce the weight of the movable unit to much degree, thus permitting an access time, a power dissipation in access, and the like to be lessened. FIG. 1 is a schematic view showing an example of such a separation type optical system. A laser beam 3 emitted from an optical pickup fixing unit 2 fixed to a base plate 1 of the apparatus is incident to an optical system movable unit 4 movable in a direction indicated by an arrow H. The direction of the incident beam is changed by a beam directing prism 7. The direction changed beam thus obtained is converged by an objective lens 5 controlled by an actuator 6, on a disk 8. Meanwhile, in such a separation type optical system, the transferring direction (feed direction) of the optical system movable unit must correspond exactly with the optical axis direction of the laser beam emitted from the optical system fixing unit. As a result, the optical path design of the optical pickup is considerably limited. FIG. 2 is a plan view showing the arrangement of an example of a pickup for an opto-magnetic disk employing such a separation type optical system. In FIG. 2, a laser beam emitted from a semiconductor laser 10 is, at the time of playback, subjected to high frequency modulation at a high frequency superposition circuit 11, and is changed to a parallel beam at a collimator lens 12. The parallel beam thus obtained is then incident to a beam shaping prism 13. Since the laser beam emitted from the semiconductor laser 10 is substantially elliptic in cross section, it is shaped by the prism 13 so that its cross section is substantially circular. A laser beam emitted from the prism 13 is incident to a beam directing prism 15 through a beam splitter 14, and is reflected thereat. The reflected beam is incident to an objective lens 16. The objective lens 16 focuses and irradiates the laser beam onto an opto-magnetic disk 17.
Hereinafter, the light polarized in a direction parallel with a p-plane which includes the normal to the splitting plane and an optical axis of incident light, is designated as "P-polarized light" and the light polarized in a perpendicular direction with the p-plane, is designated as "S-polarized light".
A reflected beam from the opto-magnetic disk 17 is incident to the beam splitter 14 through the objective lens 16 and the directing prism 15, and is reflected on the splitting plane 14a thereof. The reflected beam thus obtained travels toward a signal detection system (not shown). At this time, when attention is drawn to the splitting ratio on the splitting plane 14a of the beam splitter 14, since an emitted beam from the laser serves as a P-polarized light with respect to the splitting plane, the intensity of a reproduced or playback signal becomes maximum by allowing the ratio between a transmission factor T.sub.p of the P polarized light and reflection factor R.sub.p of P polarized light to be 67:33 and by allowing the ratio between transmission factor Ts of S polarized light and reflection factor Rs of S polarized light to be 0:100. When such an arrangement is employed, the optical axis 13A of a laser beam emitted from the prism 13 becomes in correspondence with the transferring direction V of the optical system movable unit. Thus, a separation type optical system can be constituted.
On the other hand, in the separation type optical system, the optical system fixing unit would protrude at least in one direction from a projected planform of the disk or a cartridge for sealing the disk therein as apparent from FIGS. 1 and 2 for reasons described below: (a) the movable unit and the fixing unit do not collide with each other even in the case where the movable unit is transferred to the outermost circumference of the disk, (b) adjustment of the optical system can be made even in a disk mounted state, and (c) as previously described, the transferring direction of the movable unit must correspond with the optical axis of the laser beam emitted from the optical system fixing unit. For example, in FIG. 2, the fixed unit 18 protrudes in a direction of the transferring direction V relative to the cartridge 19. Accordingly, as apparent from FIG. 2, in the case of an opto-magnetic disk drive apparatus using such a separation type optical system, the length of the optical system composed of the movable unit and fixing unit, in the transferring direction V of the movable unit cannot be shorter than a length obtained by adding a protrusion length L of the fixing unit to a diameter of the disk (or a length in the transferring direction V of the cartridge). On the other hand, the length of the drive unit in a direction W perpendicular to the transferring direction V cannot be less than the length in the direction W of the disk (cartridge). As stated above, there exist dimensional restrictions in the construction of the entirety of the apparatus.
FIG. 3 shows an optical system fixing unit of the same structure as that of FIG. 2. Furthermore, FIG. 4 shows an arrangement such that a laser beam emitted from the beam shaping prism 13 is incident to the beam splitter 14 from a direction normal to that of FIG. 3, and that a laser beam reflected on the splitting plane 14a of the beam splitter 14 is incident to the movable unit. In FIGS. 3 and 4, the same reference numerals are attached to portions corresponding to those of FIG. 2, respectively. An emitted beam which does not undergo beam shaping of existing high output semiconductor lasers is substantially elliptical in cross section, and the aspect ratio is about 3:1. Accordingly, for shaping this cross section to be substantially circular, it is desirable to set an incident angle .theta..sub.1 to the prism and an incident angle .theta..sub.2 to about 20 degrees and about 39 degrees, respectively.
When a comparison between arrangement of FIGS. 3 and 4 is made, an excessed length L of FIG. 4 in the movable unit transferring direction (indicated by an arrow V) can be clearly shorter than that of FIG. 3 in the same direction as described above.
On the other hand, a design may be made such that the minimum value in a direction normal to the transferring direction V of an opto-magnetic disk drive apparatus employing the above-mentioned optical system is limited by the length W of the cartridge, but is not dependent upon the length W.sub.1 and W.sub.2 of the fixing unit.
However, when an arrangement as shown in FIG. 4 is employed, the splitting ratio of the splitting plane 14a of the beam splitter 14 has a relationship opposite to that in the case of FIG. 3. Namely, the ratio between transmission factor Rp of P-polarized light is expressed as 33:67, and the ratio between transmission factor Ts of S-polarized light and reflection factor Rs of S-polarized light is expressed as 100:0. As a result, the reflection factor of P-polarized light with respect to the splitting plane 14a becomes higher than the reflection factor of S-polarized light with respect to the same.
As shown in FIG. 5, since the reflection factor of S-polarized light is higher than that of P-polarized light at all times independent of the incident angle, it is difficult to manufacture a beam splitter having such a splitting ratio.
In the case of recording information onto a direct read after write erasable optical disk, or erasing recorded information, an energy larger than that at the time of playback is required. To meet this, a high output semiconductor laser is used as a light source.
FIG. 6 is a plan view showing the arrangement of a different example of a conventional pickup device using a semiconductor laser.
In this figure, reference numeral 21 is a high output semiconductor laser for emitting a laser beam, and reference numeral 22 is a collimator lens for changing a laser beam emitted from the semiconductor laser 21 to a parallel beam. Furthermore, reference numeral 23 is a prism for shaping the cross section of a laser beam, and reference numeral 24 is a beam splitter having a splitting plane 24A.
Reference numeral 25 is a mirror to reflect a laser beam emitted from the beam splitter 24 in a direction perpendicular to the plane of the drawing to allow it to be incident to an objective lens 26. Reference numeral 29 is a casing for accommodating respective components.
In addition, reference numeral 27 is a unit affixed on a surface 28 of the semiconductor laser 21. This includes a high frequency superposition circuit therein.
A laser beam emitted from the semiconductor laser 21 is incident to the collimator lens 22, and is changed to a parallel beam. This parallel beam is then incident to the prism 23. A laser beam emitted from the semiconductor laser 21 is substantially elliptical in cross section. Accordingly, this laser beam is shaped by the prism 23 so that it is substantially circular in cross section.
A laser beam emitted from the prism 23 is incident to the mirror 25 through the beam splitter 24, and is reflected thereat. The reflected beam is incident to the objective lens 26. The objective lens 26 converges and irradiates the incident laser beam onto an optical disk (not shown).
A reflected beam from the optical disk is incident to the beam splitter 24 through the objective lens 26 and the mirror 25, and is reflected on the splitting plane 24a. The reflected beam is incident to a photodiode (not shown).
By driving the semiconductor laser 21 in correspondence with a recording signal, a laser beam emitted from the laser 21 is modulated in correspondence with the recording signal. Thus, information is recorded onto the optical disk.
The level of a reflected light beam from the optical disk changes in correspondence with a recording signal on the optical disk. Accordingly, a reproduced signal from the optical disk can be provided by an output from the photodiode.
The high output semiconductor laser 21 undergoes, at the time of reading signals (when its output is at low level), influence of a return beam noise, so the S-N ratio is apt to be deteriorated. In order to suppress the influence of this return beam noise, at the time of playback (at the time of low level), a high frequency component from a superposition circuit provided in the unit 27 is superimposed on a drive signal for the semiconductor laser 21.
As stated above, in the case of shaping a laser beam emitted from the semiconductor laser 21 by means of the prism 23, an optical axis 21A of a laser beam emitted from the semiconductor laser 21 and an optical axis 23A of a shaped laser beam form a predetermined angle. This angle is determined by an aspect ratio of a beam emitted from the semiconductor laser 21, a reflective index of the prism, and the like.
It is now assumed that the optical axis 23A is set perpendicular (or in parallel) to the transferring direction of the optical pickup device (radial direction of the optical disk), that one plane surface 29B of the casing 29 is arranged in parallel to the optical axis 23A, and that the outer plane surface 29A is arranged perpendicular to the optical axis 23A. In the case where no unit 27 is attached, the length in a direction perpendicular to the optical axis 23A is L.sub.11, and the length (width) in a direction in parallel to the optical axis 23A is W.sub.11.
On the contrary, in the case where the unit 27 of substantially rectangular parallelepiped is attached onto the plane 28 of the semiconductor laser 21 exposed from the casing 29, as indicated by broken lines in FIG. 6, this state is equivalent to the state where the plane surface 29A and 29B are moved so that they are in correspondence with the plane surface 30A and 30B, respectively. As a result, the length and the width of the casing become equal to L.sub.12 and W.sub.12, respectively. It is clear that the length L.sub.12 and the width W.sub.12 become larger than the length L.sub.11 and the width W.sub.11, respectively.
For the above reason, the optical pickup device, especially its drive unit is disadvantageously enlarged.